This invention relates to determining the presence of Internet Protocol (IP) multicast routers, and more particularly to determining the presence of the routers using Internet Group Management Protocol (IGMP) query messages.
Network systems such as the Internet are often configured as point-to-point or unicast systems. In these systems, a message is converted into a series of addressed packets that are routed from a source node through a plurality of routers to a destination node. In many communication protocols, the packet includes a header that contains the addresses of the source and the destination nodes. The header may also contain a sequence number that specifies the packet's order in the message.
In general, these systems do not have the capability to broadcast a message from a source node to all the other nodes in the network because such a capability is rarely of much use and could easily overload the network. However, there are situations where it is desirable for one node to communicate with some subset of all the nodes. For example, multi-party conferencing capability analogous to that found in the public telephone system and broadcasting to a limited number of nodes are of considerable interest to users of packet-switched networks. To satisfy such demands, packets destined for several recipients have been encapsulated in a unicast packet and forwarded from a source to a point in a network where the packets have been replicated and forwarded on to all desired recipients. This technique is known as Internet Protocol (IP) multicast tunneling.
Recently, routers have become available which may route the multicast addresses provided for in communication protocols such as Transmission Control Protocol/Internet Protocol (TCP/IP) using a similar technique referred to as IP multicasting. A multicast address is essentially an address for a group of host computers that have indicated the desire to participate in that group. Thus, a multicast packet may be routed from a source node through a plurality of multicast routers to one or more devices receiving the multicast packets. From there the packet is distributed to all the host computers in the multicast group. Therefore, the IP multicasting conserves bandwidth by forcing the network to do packet replication only when necessary. This offers an attractive alternative to unicast transmission for the delivery of network ticker tapes, live stock quotes, multiparty video-conferencing, and other related applications.
In order to further reduce the data traffic and increase the bandwidth of the network, the source node should avoid routing the multicast packets to ports where there are no receiver nodes or routers. However under current protocols, a means for detecting the presence or absence of the multicast routers is lacking. Thus, the Internet Engineering Task Force (IETF) is currently developing a special protocol that allows determination of the presence of the IP multicast routers.